Cargo aircraft, typically, have upswept afterbodies in order to facilitate the loading and unloading of cargo via the aft cargo door. The aerodynamic flowfield generated by the downwash from the lifting wing, usually positioned ahead of the upswept body, in combination with that of the body itself, is highly three-dimensional in form and can produce excessive aerodynamic drag forces on the aircraft. Both lift and drag forces produced on the afterbody by this complex flow field are derived from frictional effects on the body surface due to the viscous nature of the air, pressure forces which are governed primarily by the flow of air around the body shape, and the ability of a viscous boundary layer adjacent to the body surface to remain attached. With a large amount of afterbody upsweep, these basic aerodynamic properties can combine in such a manner so that a strong vortex system is shed into the wake producing yet another aerodynamic force, vortex drag, which is a characteristic aerodynamic phenomenon associated with the highly upswept afterbody. The onset of vortex drag is usually accompanied by boundary layer separation on the body surface which further compounds the aircraft drag problem.
The development of frictional forces on aerodynamic surfaces is confined chiefly to the action of the air within the boundary layer which is an extremely thin layer of fluid adjacent to the solid surface. Essentially, at the surface itself, the air is at rest, while immediately above successive layers slide over each other thereby exerting a a frictional drag force that is proportional to the viscosity of the flowing medium. Aerodynamically. it is desirable to keep the boundary layer attached to the surface thereby precluding an additional pressure drag force attendant to the separation of the boundary layer from the surface.
The tendency for the boundary layer to remain attached or to separate from the body surface is governed by the distribution of surface pressures on the body itself. A positive pressure gradient, i.e. an increasing rate of change of pressure, along with viscous drag, tends to slow the fluid motion and over an extended length of contact with the surface, the innermost portion of the fluid can be brought to rest at the separation point. In fact, from points downstream, a reverse flow back towards the separation point can be produced. For the upswept afterbody, this aerodynamic effect results in a breaking away of the streamlines from the body aft-surface leaving an area of low pressure, i.e. pressures drag, between the two lines of separation points on either side of the body. If the shape of the body and its basic flow field is inherently disposed toward producing a vortex wake, the breakaway streamlines will roll up into a vortex pair that trail downstream with significant amounts of wasted energy contained in the rotational flow.
To best describe the disposition of the upswept afterbody for producing a vortex-dominated wake, an analogy can be made to a low aspect ratio (small span relative to the chord length) three-dimensional wing. Under lifting conditions. a strong vortex trails from both wing tips as a consequence of the pressure differential existing between the wing upper-surface (low pressure) and the lower-surface (high pressure). At the wing tip, the air flows from the high pressure region around the tip surface towards the lower pressure creating a rotational flow field (vortex) which trails off into the wake. The upswept afterbody can be considered as aerodynamically analogous to an inverted, exceptionally low aspect ratio wing trailing a vortex pair and with negative lift produced by the positive upsweep of the body aft end (negative camber in terms of the wing analogy). For cargo aircraft, the negative lift produced by the upswept fuselage must be recovered by the main wing by increasing the angle of attack of the wing-body combination. This process produces yet another detrimental drag force on the system--an increase in wing induced drag which would not be necessary except for the negative lift produced by the fuselage upsweep.
With the intent of reducing the foregoing combination of frictional, pressure, vortex and induced drags characterizing the upswept fuselage, a wide variety of devices have been employed. These would include strakes, fences, blowing jets, vortex generators and body reshaping. Some success has been achieved with strakes attached to the afterbody lower surface so as to change the position, character and strength of the shed vortex. To date, such drag reductions have been only partially successful in offsetting the potential levels of drag known to be present in the total wing- body-vortex system.
As a related subject of research, attempts have been made to reduce the base drag of bluff, axisymmetric bodies, such as circular cylinders with cut-off or conical bases, by using longitudinal grooves cut lengthwise into the body surface. Results have been reported on by Howard, F. G., Weinstein, L. M. and Bushell, D. M. in "Longtiduinal Afterbody Grooves and Shoulder Radiusing for Low-Speed Bluff Body Drag Reduction"; ASME Winter Annual Meeting, Washington, D, C., Nov. 1981. Base drag for three-dimensional bodies is primarily a function of the friction drag of the forebody, the subsequent thickening of the boundary layer as the base is approached, and the severity of the angle of cutoff of the base. Shoulder radiusing, boat-tailing and in the instance cited longitudinal grooves, are recognized aerodynamic concepts which can provide some drag relief for bluff bodies exhibiting high levels of base drag. One problem experienced with the longitudinal grooves, however, has been that the effectiveness of the grooves tends to diminish with increasing Reynolds number thus negating their value at full scale flight conditions. Additionally, attempts to influence the drag of an upswept afterbody by the addition of simple rectangular or "V"-shaped grooves placed both longitudinally or at an angle to the local streamlines have not been successful and, in most cases, a drag increase has been noted.